Full-motion video disc with reference information for slow-motion or freeze playback

ABSTRACT

An optically readable disc like a compact disc contains a sequence of full-motion video scene digital video blocks, having differing lengths. Each video block contains the entire encoded picture information for at least one picture of the sequence. A header in each video block contains reference information referring to the location of a predetermined video block in the series of blocks, so that this information permits a playback device to recover from a slow-motion or freeze mode by reading from a desired location in the predetermined video block.

This is a division of application Ser. No. 08/269,941, filed Jun. 28,1994 which is a continuation of Ser. No. 07/707,527 filed May 30, 1991,now abandoned.

FIELD OF THE INVENTION

The invention generally relates to a method of transmitting a finitesequence of pictures of a full-motion video scene in a digital formatvia some transmission medium. More particularly, the transmission mediumis constituted by a compact disc-like medium.

The invention also relates to a display device in which the picturesthus transmitted are processed and made suitable for display on adisplay screen, as well as to an optically readable medium on which saidpictures are stored.

DESCRIPTION OF THE PRIOR ART

More than fifteen years ago the firm of Philips marketed an opticallyreadable disc on which not only audio signals but also analog videosignals were recorded. This disc was referred to as video long-play(VLP) and it supplemented the well-known audio long-play (ALP). Ascompared with video tapes, such optically readable discs have theadvantage that their quality does not deteriorate due to repeated use.However, as compared with video tapes they have the drawback that theyare not re-recordable.

In the last ten years there has been a completely new trend, namely thatof the optically readable audio discs generally known under the name ofCD audio (Compact Disc audio). Due to its general acceptance and theever increasing demand for integration of audio and video apparatuses, acompact disc video has been created on which not only digitized audiosignals but also an analog video signal is recorded which corresponds toa full-motion video scene of several minutes' duration.

To be able to extend the duration of such a video scene, the analogvideo signal is digitized. A full-motion video scene is then consideredas a finite sequence of pictures of which, for example, twenty-five orthirty occur each second. Such a picture comprises, for example, 288picture lines with 352 pixels for each line. With the aid of somesuitably chosen encoding algorithm the sequence of pictures is convertedinto a series of video blocks each of which comprises such an amount ofdigital information that each pixel of a picture or of a predeterminednumber of pictures can be reconstructed. The most efficient encodingmethods convert the picture sequence into a series of video blocks ofdifferent lengths (i.e. distinct numbers of bits). Consequently, thestart of a new video block is not predictable. It is particularly thisfact and the fact that the disc rotates at a predetermined constantspeed which make features such as slow motion and freezes not very wellpossible. If it is assumed that the user freezes a picture for oneminute while the disc continues its normal rotation, 25×60=1500 pictureswill not be displayed during this minute when the normal display of thesequence takes place at a picture frequency of, for example, 25 picturesper second. Thus, this causes a discontinuity in the display of thepicture sequence.

SUMMARY OF THE INVENTION

It is an object of the invention to contribute to the above-mentionednovel development by obviating the above-mentioned drawback.

According to the invention a header is added to each video block, saidheader comprising reference information referring to a predeterminedvideo block in the series of video blocks.

This predetermined video block may be the actual video block but alsothe next video block in the series. The reference information mayrepresent, for example, the ordinal number of said video block in theseries.

With the use of the measure according to the invention a display devicecan be adapted to read a video block including its header added thereto,to separate the header from the actual picture information and to storeit in an allocated memory location in a memory. The actual pictureinformation is applied to a decoding circuit which determined the Y, Uand V or R, G and B values for each pixel of a picture to be displayedand temporarily stores them in a picture memory. Whenever the picturestored in this picture memory must be displayed, it is read line by lineand location after location. In the case of normal display of a picturesequence, the contents of the picture memory are refreshed, for example,every 1/25th second. In the case of slow motion this picture memory is,however, is refreshed, for example only five times a second, whereas inthe case of a freeze the picture memory is not refreshed until after thenormal display of the picture sequence has to be continued again.Whenever the contents of the picture memory are to be refreshed, thereference information associated with the picture currently stored inthe picture memory is read and the video block to which reference ismade is searched on the medium, read, processed and displayed as apicture. Since the indicated video block is the actual or the next videoblock in the series, the display of the picture sequence is each timecontinued without any discontinuity.

Since it takes some time to process the information read from the discand since the video blocks occur at irregular instants due to theirdifferent lengths, it is advantageous to use a time code as referenceinformation which is a measure of the instant when the relevant picturemust be displayed relatively to a reference instant in the case ofnormal display of the picture sequence. This time code comprises anindication in hours, minutes and seconds, but also an auxiliary ordinalnumber n mod fr indicating the ordinal number of a picture in a seriesof pictures to be displayed within the period of a specific second. Moreparticularly, n refers to the previously mentioned ordinal number of thevideo block and fr refers to the frame frequency.

As is generally known, the display device comprises a drive circuit fordriving the read apparatus reading the information from the medium. Thisdrive circuit can drive the read apparatus in such a way that it willexactly take that position which corresponds to said time code. However,since the video blocks have different lengths, this position will ingeneral not correspond to the position of the video block comprising thepicture information of the desired picture. In other words, when duringslow motion or after a freeze the read apparatus is exactly positionedagain in accordance with the time code, there will still be adiscontinuity in the picture display. To avoid this, the referenceinformation not only comprises a time code characterizing a location onthe medium, but also a pointer code pointing at the location where thebeginning of the video block of the desired picture can be found. Moreparticularly, this pointer code represents the difference in locationbetween the location of the beginning of the relevant video block on themedium and the location characterized by the time code.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows diagrammatically the general structure of a display devicefor video pictures stored in a digital form on a compact disc-likemedium;

FIG. 2 shows diagrammatically a compact disc-like medium having a trackwith a sectoral division;

FIG. 3 shows in some diagrams different types of headers which can beassociated with the video blocks;

FIG. 4 shows diagrammatically an encoding method for full-motion videopictures and the composition of the video blocks which are transmitted.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows diagrammatically the general structure of a display deviceadapted to receive digitized pictures of a full-motion video scene whichare transmitted by means of a compact disc-like medium (hereinafterreferred to as disc). This display device has a read apparatus 1 bymeans of which information present on the disc 2 can be read andconverted into a data current b. This data current is applied to aninterface circuit 3 which in its turn applies signals to a CD drive unit4 controlling the movement of the read apparatus 1 with respect to thedisc 2. The interface circuit 3 is further connected to a centralprocessor unit 5 having an internal memory in which the conventionalsystem software is stored to realise the base drive of the displaydevice. This central processor unit 5 receives the data current b viathe interface circuit 3 and possibly applies drive commands for the CDdrive unit 4 to the interface circuit 3. An auxiliary processor 6 may beconnected to the central processor unit 5 via a two-way communicationchannel in which, for example, specific software present on the disc canbe processed; the so-called application software. In order to enable auser to influence the operation of the display device, the centralprocessor unit 5 also has a unit for user's commands connected to it,for example, a keyboard 7.

For displaying the different information signals, a full-motion videodecoder 8 and an audio processor 9 are connected to the centralprocessor unit 5 via a two-way information channel. Audio processor 9processes the digital audio signals on the disc and applies them in ananalog form to a loudspeaker 10. The full-motion video decoder processesthe digital video signals on the disc and applies them in an analogform, for example, as primary colour signals R, G and B to a monitor 11.As a means for the display of the digital video signals, a picturememory 12 is connected to the full-motion video decoder, in which memorya digital R, G and B value may be stored after each pixel has beendecoded, which values are converted into analog signals, when the memoryis being read, and applied to the monitor 11.

FIG. 2 shows diagrammatically at A a part of the track of a compactdisc-like transmission medium. A so-called sector, of which there may beapproximately 300,000, is present between two consecutive points a, b,c, d, e and so forth. The structure of such a sector is showndiagrammatically at B in FIG. 2. It comprises, for example 2352 bytesand is divided into a sector header H comprising 24 bytes and a datafield DF comprising 2328 bytes.

The sector header H comprises a synchronization word SYNC.1 of 12 bytes,a sector number SCTNR of four bytes and a service word SW of eightbytes. The synchronization word SYNC.1 marks the beginning of a sectorand is recognized as such by the central processor unit 5. It comprisesone byte consisting exclusively of O-bits, followed by ten bytesconsisting exclusively of 1-bits and finally again one byte consistingexclusively of O-bits. The bytes of the sector number SCTNR indicate theordinal number of the sector on the disc. This ordinal number is read bythe central processor unit 5 and stored in an internal memory. Thecentral processor unit 5 therefore exactly recognizes from which sectorthe last-received information originates. The service word SW indicateswhether the sector is a video sector, an audio sector or a computer datasector. The central processor unit 5 applies the data field of acomputer data sector to, for example, the auxiliary processor 6, thefield of an audio sector to the audio processor 9 and the field of avideo sector to the full-motion video decoder 8.

A part of the track of the disc is shown once more at C in FIG. 2 inwhich the sectors are now indicated as blocks SCT. More particularly,the video sectors are shaded. As has been attempted to express in theFigure, the video sectors and the other sectors (audio and computer datasectors) occur in an arbitrary sequence.

As is shown at B in FIG. 2, the data field DF of a sector is dividedinto data slots DS. Each data slot in an audio sector comprises, forexample, a 16-bit audio word of a digital audio signal. Each data slotin a video data sector and in a computer data sector comprises one 8-bitvideo word of a digital video signal and one data byte (8-bit),respectively.

As stated in the foregoing, a video picture comprises, for example, 288lines of 352 pixels each and may thus be considered as a matrix ofpixels P(i,k), where i=1, 2, . . . 288 is the ordinal number of the lineand k=1, 2, . . . 352 is the ordinal number of the pixel on this line(column). The colour of a pixel is entirely determined by a luminancevalue Y(i,k) and two colour difference values U(i,k) and V(i,k). Ifthese three values with eight bits of each of these pixels wereaccurately encoded, approximately 130 video sectors would be requiredfor one picture. Since the conventional rotational speed of the disc issuch that 75 sectors pass the read apparatus every second, more than onesecond is required to obtain the information of all pixels of a picture.

Many encoding methods are known to reduce the number of bits per pictureand hence the number of sectors required per picture without anydeterioration of the picture quality. The most efficient encodingmethods convert consecutive pictures of a picture sequence into videoblocks having distinct numbers of bits.

For the purpose of illustration FIG. 3 shows at SEQ a picture sequencecomprising the pictures B₀, B₁, B₂, . . . B₆ which jointly represent afull-motion video scene. By subjecting this picture sequence to somesuitably chosen encoding method, picture B_(n) is converted into a videoblock VB_(n) (n=0, 1, 2, . . . 6). It will be assumed that its bitsoccur serially. Distinct video blocks will generally have distinctlengths. The series of video blocks thus obtained is shown at VBS inFIG. 3.

To be able to distinguish the different video blocks, a block header isadded to each of them. The block header which is added to the videoblock VB_(n) is denoted by VBH_(n) in FIG. 3. It comprises a Start ofFrame code SOF followed by a synchronization word SYNC.2 which in itsturn is followed by a further Start of Frame code SOF. Moreparticularly, the hexadecimal code "C000" is chosen for the Start ofFrame codes and the hexadecimal code "0002" is chosen for thesynchronization word SYNC.2. The central processor unit 5 candistinguish the consecutive video blocks by means of these codes and canrealise word synchronization so that the display of the picture sequenceat a predetermined picture frequency is ensured in the case of normaluse.

Since the disc rotates at a constant speed when the picture sequence isbeing displayed, features such as slow motion and freeze are not longerpossible when the above-described transmission method is used, becausein such cases the synchronization between reading the information of thedisc, decoding this information and displaying the pictures representedby this information is lost. Let it be assumed that the user freezes apictures for one minute. As from the instant when the normal display ofthe picture sequence is continued, 60×75=4500 sectors pass the readapparatus. Although the decoder 8 has a buffer for storing the datauntil the instant when their display is desired, the capacity of thisbuffer will be considerably smaller than is required for storing these4500 sectors. This means that a part of the information which has beenread is lost in the case of a freeze but also in the case of slowmotion, which leads to discontinuity in the display of the picturesequence. To inhibit this and to be able to restore the necessarysynchronization, the block header also includes a reference field REFwhich comprises reference information referring to a predetermined videoblock, for example, that of the actually displayed picture and/or thatof the next picture in the picture sequence. The central processor unit5 temporarily stores the contents of this reference field in a memoryallocated for this purpose. These contents are preserved until asubsequent video block of the disc has been read and processed. After aslow motion or freeze feature has been switched off, the centralprocessor unit 5 supplies drive commands to the CD drive unit 4, therebymoving the read apparatus 1 in such a way and for such a long time untilthe video block to which reference is made has been found.

This reference information can be composed in different manners. Forexample, as is indicated at REF.1 in FIG. 3, this reference informationmay comprise the ordinal number n of the actually displayed picture inthe picture sequence. This means that when the display of the picturesequence after a freeze is resumed, the picture sequence is continued atthis frozen picture. The central processor unit 5 may, however, also beprogrammed in such a way that it automatically adds one unit to thisordinal number n when the display of the picture sequence is resumed sothat the CD drive unit 4 will search the video block with referenceinformation n+1. It will be evident that in such a case it is preferableto take the ordinal number n+1 of the next picture as referenceinformation.

If consecutive video blocks on the disc are enumerated in conformitywith the series of natural numbers, the ordinal number n of each videoblock will have to be checked on conformity with the desired associatedordinal number when the desired video block is being searched. Thisresults in a rather slow search procedure. Another possibility is showndiagrammatically at REF.2 in FIG. 3. The reference field REF does notonly comprise the ordinal number n of the actual or the next video blockVB_(n), but also the sector number SCTNR(VB_(n)) in which this videoblock starts. Since all sectors have equal lengths and since 75 of themcan be read every second, the sector of a specific number can besearched considerably more rapidly than a video block having a specificordinal number.

A third possibility is shown at REF.3 in FIG. 3. Here, the referencefield does not only comprise the sector number SCTNR(VB_(n)) and theordinal number n, but also the sector number SCT(VB_(n+1)) of the nextvideo block. The central processor unit 5 may be adapted in such a waythat it computes the ordinal number n+1 of the next video block and theCD drive unit moves the read apparatus to the specific sector in whichit searches the beginning of the video block having the computed ordinalnumber.

In view of the large number of sectors and the large number of videoblocks which may be present on the disc, the possibilities shown atREF.1, REF.2 and REF.3 will comprise many bits. The reference field REFtherefore preferably comprises a time code TC which indicates withrespect to a specific reference instant (for example, the start of thesequence) when the corresponding picture must be displayed. As is shownat REF.4 in FIG. 3, this time code is preferably expressed in hours H,minutes M, seconds S, as well as a number n mod fr indicating theordinal number of the picture in the Sth second. Here, fr denotes theframe frequency. The relationship between this time code and the sectornumber will be elucidated with reference to the following example. Letit be assumed that the following time code has been allocated to a videoblock which is read at the instant t_(o) (see FIG. 1C) of the disc:

H=00

M=00

S=03

n mod fr=10

If the rotational speed of the disc is such that 75 sectors pass theread apparatus 1 every second, a determinable number of sectors haspassed the read apparatus at the instant when the relevant picture isdisplayed. In fact, 3*75=225 sectors pass in the S=3 seconds. Startingfrom a frame frequency fr of 25 Hz, 10 pictures correspond to 10/25seconds, i.e. 75*10/25=30 sectors. In other words, a total number of225+30=255 sectors has passed the read apparatus as from the referenceinstant. If this picture is frozen, the disc will continue to rotate andthe read apparatus will continue to follow the track. To be able toresume normal display of the picture sequence, the central processorunit 5 may be programmed so as to drive the read apparatus 1 in such away that it searches that video block on the disc whose time codecorresponds to the time code of the freeze.

Since the video blocks have different lengths and a specific period oftime is required to decode the picture information in a video block,there is usually no unambiguous correlation between the instant when apicture is displayed and the instant when the corresponding video blockof the disc is read. For example, it may occur that the correspondingpicture of a video block read at an instant t₁ (see FIG. 2C) of the discis not displayed until an instant t₂. Thus, this time code does notindicate at all where the relevant video block on the disc can be found.This video block can only be found by comparing the time codes of allvideo blocks located in the vicinity of the sector (255) to which thetime code of the actual picture corresponds. Such a search procedure isusually very slow. It is therefore advantageous to incorporate a pointercode PC in addition to this time code TC in the reference field, whichpointer code provides an indication about the location where thebeginning of the video block corresponding to the picture to bedisplayed as the first picture after the freeze can be found on thedisc. As has already been noted, this pointer code can be used toindicate the freeze picture itself, but also to indicate the nextpicture in the picture sequence.

This pointer code may have many shapes, but preferably the shape shownat REF.5 in FIG. 3. It comprises a number SCTOFFST (sector offset) whosesign and magnitude represents a correction of the sector numbercorresponding to the time code TC, and a number WRDOFFST (word offset)indicating the ordinal number of the word in the sector with thecorrected sector number representing the first SOF word of the desiredvideo block. The following example will be given for the purpose ofclarification. Let it be assumed that the time code TC is again equal tothe time code in the above-given example and thus corresponds to sectornumber 255. Let it be assumed that SCTOFFST=-20 and WRDOFFST=16. Thestart of the next video block which must be read from the disc thencorresponds to the sixteenth word in sector 255-20=235.

It has been assumed in the foregoing that each picture B_(n) isconverted into one video block VB_(n) by means of some encoding process.Such an encoding process may, however, be chosen to be such that a videoblock comprises video information of a number of consecutive pictures. Avery advantageous encoding process is the so-called hierarchic encodingwhich will be further described with reference to FIG. 4. To this endthe picture sequence of the pictures B₀, B₁, B₂, . . . B₆ is again shownat SEQ in FIG. 4. This sequence is divided into a number ofsub-sequences and an order is allocated to each sub-sequence. In thecase considered here, the picture sequence is divided into twosub-sequences. The first sub-sequence, which will be referred to aszero-order sub-sequence, comprises the even-numbered pictures B_(2m)(m=0, 1, 2, 3). The second sub-sequence, which will be referred to asfirst-order sub-sequence, comprises the odd-numbered pictures B_(2m+i).

The picture B₀ is converted by means of intraframe encoding into anintraframe encoded picture DB₀ which comprises all information requiredto reconstruct the picture B₀ and which will therefore be referred to assequence access block. The other pictures of the zero-order sub-sequenceare subjected to an interframe encoding. More particularly, a system ofmotion vectors Q_(2m),2(m+1) is determined in this example for eachpicture of this zero-order sub-sequence. Each vector of this systemdescribes the direction and the distance along which a pixel or a groupof pixels of the picture B_(2m) must be displaced so as to reach(approximately) the location occupied by this pixel or group of pixelsin the picture B₂(m+1). Starting from the picture B_(2m) a predictionpicture B'₂(m+1) is computed for the picture B₂(m+1) by means of thissystem of motion vectors Q_(2m),2(m+1). A zero-order difference pictureDB₂(m+1) is obtained by difference production of B₂(m+1) and B'₂(m+1).

The pictures of the first-order sub-sequence B_(2m+1) are also subjectedto an interframe encoding. More particularly, the pictures of thezero-order sub-sequence and the previously computed systems of motionvectors Q_(2m),2(m+1) are used in this example. The motion will hereinbe assumed to be linear. This means that the displacement of a pixel ora group of pixels in the picture B_(2m+1) with respect to the locationin the previous picture B_(2m) used as a reference is half thedisplacement of this pixel or this group of pixels in the next pictureB₂(m+1) with respect to said reference location. The following procedureis carried out for the interframe encoding of the pictures B_(2m+1) ofthis first-order sub-sequence. Starting from the previous picture B_(2m)and a system of motion vectors 1/2Q_(2m),2(m+1) each having the samedirection as the motion vectors of the system Q_(2m),2(m+1) but beinghalf as long, a first prediction picture is determined. Starting fromthe picture B₂(m+1) and a system of motion vectors -1/2Q_(2m),2(m+1)each having a direction which is opposed to the direction of the motionvectors of the system Q_(2m),2(m+1) and being only half as long, asecond prediction picture is determined. First and second predictionpictures are subsequently added together and the sum picture thusobtained is divided by two. The result is a prediction pictureB'_(2m+1). The first-order difference picture DB_(2m+1) is obtained bydifference production with the original picture B_(2m+1). Thesedifference pictures are converted into serial data blocks DB^(S) ₀,DB^(S) ₁, . . . DB^(S) ₆ by means of quantization and further encoding.Since a system of motion vectors is associated each time with twoconsecutive pictures, two serial data blocks together with theassociated motion vectors are composed to one video block VB_(n) (n=0,1, 2, . . . ). The sequence in which the data blocks and motion vectorscan be transmitted is shown at TR in FIG. 4.

We claim:
 1. An optically readable disc on which a sequence of picturesof a full-motion video scene is stored in the form of a series of videoblocks,characterized in that said video blocks have unpredictablydiffering respective lengths, each block including the entire encodedpicture information for at least one picture of said sequence, and eachvideo block includes a header comprising reference information referringto the location of a predetermined video block in the series of videoblocks.
 2. A disc as claimed in claim 1, characterized in that saidreference information comprises the ordinal number of the relevant videoblock in the series.
 3. A disc as claimed in claim 1, characterized inthat the reference information in the header of a given video blockrefers to the location of the next video block in the series.
 4. A discas claimed in claim 1, characterized in that said reference informationcomprises a time code which, with reference to a reference instant, isindicative of the instant when the picture characterized by the relevantvideo block is displayed in the case of normal display of the picturesequence.
 5. A disc as claimed in claim 4, wherein said time codecomprises a time indication in hours, minutes and seconds, and anauxiliary ordinal number indicating the ordinal number of a video blockin a series of video blocks whose corresponding pictures must bedisplayed at a frame frequency within the period of a predeterminedsecond.
 6. A disc as claimed in claim 4, in which the series of videoblocks is divided into sectors each having a sector number, and whereinthe reference information further comprises a pointer code (PC)comprising a sector offset code which, together with the time code,points at the sector number indicating the beginning of the relevantvideo block.
 7. A disc as claimed in claim 6, wherein the pointer codefurther comprises a word offset code indicating the ordinal number ofthe word in the relevant sector where the video block begins.
 8. A discas claimed in claim 1, in which the series of video blocks is dividedinto sectors each having a sector number, and wherein the referenceinformation further comprises a pointer code (PC) comprising a sectoroffset code which, together with the time code, points at the sectornumber indicating the beginning of the relevant video block.
 9. A discas claimed in claim 8, wherein the pointer code further comprises a wordoffset code indicating the ordinal number of the word in the relevantsector where the video block begins.